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Additive manufacturing (AM) is a manufacturing process of components by adding material layer upon a layer [2, 15, 16]. The term AM represents various technologies, such as Rapid Prototyping (RP), Direct Digital Manufacturing (DDM), Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF), Selective Laser Sintering (SLS) , Stereolithography (STL) and Laminated Object Manufacturing (LOM) [24]. Today, different materials, such as plastics, polymers, metals and composite materials are used in additive manufacturing.

However, structural materials with high melting temperature face many challenges and are currently expensive and less commercially used.

The history of AM starts back in 1980s [25], when Charles Hull invented stereolithography (SLA); a form of 3D printing system, STL- file format and slicing technique [24, 26]. The STL- file format and slicing technique is commonly used by the AM machines. On the late years, several technologies among them FDM developed by Stratasys are commercialized.

Furthermore, several AM technologies have been developed and commercialized. Nowadays, conventional AM is widely used manufacturing process of prototypes and specific functional parts.

AM have several advantages comparing to the traditional manufacturing methods. The design freedom, reduction of material waste and manufacturing cost are among others that make AM an attractive technology to the manufacturing industry and research community. AM allow engineers to design more complex geometries without restricted by manufacturing complexity.

Fabricating such complex geometries in conventional manufacturing method is difficult and time-consuming process. In AM process the designing and manufacturing processes takes place at same place [27]. This eliminates delivery time, part storage, transport cost and manufacturing expenses [24]. Despite all the advantages of AM, there are some drawbacks that must be solved; Parts are built layer-by-layer in AM, this means the strength of the part is weaker perpendicular to the working plane [6, 28]. This is due to the interfacial bonding between layers [6]. This kind of problem is more problematic in additive manufacturing of composite materials, where the fiber-matrix bonding is weak in addition to the AM parameters [4]. Unlike conventional manufacturing processes of composite materials, no pressure/vacuum is applied during fabrication of parts by AM process. Therefore, composites produced by AM

experience an increase in porosity and this largely weaken their property in the building direction (z-direction) [1, 4, 21].

All AM techniques operate in the similar principle. The processes of 3D printing (AM) starts with designing a geometry of a component in 3D CAD (computer-aided design) software. The file is then converted into a stereolithography file format (STL), commonly used by all AM machines. The design is then sliced into thin layers by a slicing software. The required information of the part such as layer thickness, tool path, part orientation, type of material and others are prepared in the slicing software and included in the STL-file. This STL-file is then sent to the AM machine and used as a command when fabricating the designed part.

During fabricating using 3D print, a filament is extruded through a hot nozzle at a constant rate laying thin layers of material upon each other until the designed component is completed.

The filament is pushed into the nozzle by a stepper motor pushing the melted filament out of the nozzle. The filament is heated inside the nozzle until it reaches its glass transition temperature (Tg). The 3D printer head moves only in XY-plane, printing the outer edges of the part first and then proceeds to the infill patterns. Once one layer is successfully printed, the working bed moves one step down leaving one-layer thickness for the next print. This process repeats until the entire component is printed.

Surface finish of additively manufactured parts depend on the fabrication layer thickness.

When printing curved geometries in the z-direction, the process produces a stair-steps. The raster-effect is appearing as successive layer must lay at an offset from the previous layer. This raster effect creates poor overall surface finish and strength. Typically, design orientation play an important role in additively manufacturing of components. Understanding the loading conditions of the component is required when making decision on the printing orientation, this is due to bearing capacity of parts in z-direction is often weak. The highest strength is obtained when the load bearing part lays in the xy-plane. Sometimes it can be essential to split parts into multiple printed pieces to achieve optimal strength. Identifying critical dimensions of your part is another case which require consideration, because 3D printers have higher precision in plane parallel to the working plane.

Complex inner geometries or overhangs require a support. Supports can be reduced by selecting the maximum bed contact. However, this is not always possible due to the several parameters to prioritize. One option to solve this is to used angled overhangs to reduce and improve support . Supports are printed in a pattern that is relatively easy to remove wither in solution or break out. Some AM machines have a separate material only used to print the support. However, Mark-Two has not a separate support material, instead the plastic matrix material is used to build the support and the matrix phase of the part. The support is printed in some way it is easily to remove.

AM have great advantages over conventional manufacturing process and previously tedious complex geometries can be manufactured easily using AM. AM requires less raw material for manufacturing of same component compared to subtractive manufacturing processes. Since components can be additively manufactured at sites, time of delivery, undesired mass and storage of excess parts can significantly reduced. AM reduces additional machine requirements

since from start-to- finish manufacturing is provided by the 3D printer. However, AM fabricated parts have anisotropic [29] material property due to the weaker bonding strength between adjacent printed layers both in Y- and Z-direction[6, 28], and are used for fabricating parts with known loading conditions .

Thermoplastic materials ABS, PLA and Nylon are commonly used filaments in 3D print [29], but these materials have limited stiffness/strength and cannot applied to conventional engineering applications. However, strengthening these polymers with continuous fiber reinforcement (CFR) has provided significant increase in stiffness/strength of the components[4, 5]. Dickson et al, in study reported a tensile strength comparable with engineering materials such as aluminum 6061-T6[21]. Additively manufacturing of Continuous Fiber Reinforced Polymers (CFRP) using 3D machines is relatively new method and it is at its early stage. Moreover, understanding mechanical properties of additive manufactured materials is still limited [5].

Table 2. Few Advantages of Additive manufacturing.

Advantages of AM Explanation

Design freedom Complex geometries can be manufactured with minimum limitation

Material efficiency Reduced material waste Weight reduction of

parts

Giving a part strength only at required functionality and reducing unwanted mass

Reduce storage No need to have spare parts at store Low manufacturing

cost

No need for additional machines and operators and low product development cost

The mechanical properties of components produced by AM is dependent on several factors [8], such as the building direction [5], thickness of layer [3, 6], bonding strength between layers [6], formation of voids [21] and type of filament material [6]. The effect of printing parameters such as infill-speed, nozzle temperature and layer thickness has studied by Ning et al, [3].

Results indicated infill-speed of 25 mm/s, nozzle temperature of 220°C and thicker layers led to largest average stiffness/strength [3]. Ning et al. also concluded high nozzle temperature increases porosity of composites and reduce their strength [21].

Generally, in composite materials both the AM parameters and fiber orientation influence the anisotropic property [30]. A unique advantage of AM when used for composite materials is that the orientation and alignment of continuous fiber can be accurately located in complex geometries, which is very difficult in traditional molding fabrication [3].

Finally, FFF method can be used to additively manufacture parts from several materials, such as metal parts. AM of material with high melting temperature requires special printing parts and is expensive compared to thermoplastic filament. Moreover, Powder Bed Fusion (PBF), Direct Energy Deposition (DED) are commonly used AM methods for metals.